Radio data communications systems are well known in the art. One such system is the single frequency reuse (SFR) system wherein a large geographic area is provided data message delivery service by deploying multiple fixed stations each serving a smaller coverage zone. In the SFR system the fixed stations are often inactive (not transmitting) unless and until needed, the system is coordinated by a network controller, and all radio communications are conducted on a single radio channel. The smaller coverage zones tend to overlap in part to insure service to a portable or mobile station (portable) regardless of present radio signal propagation conditions and in part because the overlap generally is unavoidable in a practical situation. Practitioners, striving to maximize capacity of the SFR radio data communications system (the amount of data message traffic that may be delivered in a given time period), have sought a high degree of reuse and concurrently shunned network overhead.
The rubric, "reuse", for example a reuse data path, implies something about the extent a given radio channel (transmit and receive frequency) is being reused at any point in time at different locations throughout the radio data communications system. Ideally, the highest degree of reuse occurs when all fixed stations, i.e. coverage zones or paths, are simultaneously and independently active, delivering data message traffic. Overhead, undesirable as such, in some sense represents system inefficiencies and comes in varying flavors including, data transport overhead such as synchronization blocks and error coding overhead, system management overhead such as control type interactions with portables, and, effectively, overhead resulting from unsuccessful and therefore subsequent attempts to deliver a data message. Various practical considerations limit the degree of reuse that may be achieved as well as the minimum amount of overhead necessary to provide service.
Such practical considerations include imprecise knowledge of a particular portable location, the overlapping coverage zones, and implications necessarily following therefrom. The portable location knowledge in part defines an optimum, and likely unique, data path, hence fixed station, to be used for a successful data message delivery to a particular portable. To demonstrate, if a data message delivery is unsuccessfully attempted via an inadequate data path as a result of inaccurate portable location knowledge, system capacity may be adversely impacted. Noting perhaps the obvious, such inaccuracy may be the result of simply outdated knowledge that is further complicated by the fact that a portable's and particularly mobile's location may change noticeably over comparatively short periods of time.
Further, since all fixed stations and portables operate on the same channel the extent the ideal may be approached is constrained by the overlapping zones and possible attendant radio signal interference within these zones. Specifically when a portable is deployed in an area served by multiple fixed stations all save one of such fixed stations will normally have to be inactive before the data message may be successfully delivered. In such circumstances the adjacent non-overlapping zones ordinarily served by the inactive fixed stations will be without service during the time required to deliver the data message. To the extent that fixed stations are forced inactive the potential capacity of the system is sub-optimum. To the extent that such inactivity results from an error in the portable location knowledge the impact on capacity is compounded when and if the next attempt is directed to another such overlapping zone.
The network controller, charged with coordinating the system includes, among others, a data message routing function for selecting the appropriate path to attempt a data message delivery to a particular portable. This path selection will depend in part on an estimate of a particular portable location as well as other system activity and may include when to attempt a data message delivery, which fixed station and hence radio path to utilize, and therefore, or additionally, which fixed stations and hence paths to inactivate. The data message routing functions of present day data communications systems, striving to optimize reuse, have tended to emphasize or prioritize reuse data paths and only as a last resort, if ever, select a non-reuse data path. As used herein a non-reuse data path implies that fixed stations serving adjacent overlapping coverage zones and thus adjacent non-overlapping coverage zones are inactivated or forced to forego activation. Furthermore, in order to avoid the overhead associated with more active approaches, the estimate of a particular portable location has been based on history, specifically an evaluation of the portable location that depends on the last data communications transaction with the particular portable.
This evaluation typically depends on each fixed station assessing the portable's signal quality, e.g. signal strength, bit error rate (BER), or reasonable equivalent, as observed at such fixed stations when the portable was last active. Armed with these observations the network controller, normally relying on a weighted average of the observations, will prioritize and rank data paths that would likely result in a successful data message delivery to a portable. When a data message delivery is required the network controller relying on the historical data path rankings and other system traffic can select a data path corresponding to a particular portable for a data message delivery attempt. If the attempt is unsuccessful, i.e. the portable does not confirm (acknowledge) the delivery attempt, the next best data path may be utilized, etc. etc. Without more, there is no guarantee the data message will ever be delivered using these procedures. Clearly each unsuccessful attempt represents overhead or system inefficiency in that no useful communications has occurred and system capacity has been used.
Other considerations being equal, the likelihood that a given attempt will be unsuccessful depends on the extent the portable location estimate has become stale or obsolete. Potential problems notwithstanding, this system operates reasonably efficiently and avoids the overhead associated with more active approaches, such as continued periodic portable transmissions, so long as the average time between deliveries to a particular portable is less than the average time required for the corresponding portable location estimate to become stale. When these conditions are not satisfied, for example in a system providing occasional data message delivery service to each of a multiplicity of portables, available system capacity may degrade significantly due to the large number of repeated delivery attempts required. Furthermore, where virtual absolute delivery reliability is a requirement the above approach without more is insufficient. Clearly a need exists for data message delivery improvements in data communications systems using SFR networks.